This page offers a guide on where to locate and how to complete The Last Leaf of Fall, which is a Mystery located in Ledecestrescire, Assassin’s Creed Valhalla.
Speak to the Girl Beneath the Tree
You’ll find this Mystery southwest of Alcestre Monastery. When you speak to the girl beneath the tree, she will tell you of the promise her father made that he would return before all of the leaves fell from the tree in their yard. She is panicking because there is only one leaf left on the tree. You’ll then be given the option to reassure her or tell her to move on. The outcome is the same whichever option you choose, so pick whichever you feel is the most suitable answer.
When you’re done talking to her, you’ll need to shoot down the final leaf using your bow (a bit harsh!), so that the child moves on. Completing the quest gives you 1,200 EXP, you can also pick up the Old Leaf if you wish.